1. Field of the Invention
The present invention relates to fused-fiber wavelength division multiplexers (WDM) and, in particular, to dense WDM (DWDM).
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional wavelengths can be cost prohibitive, and electronic system components may impose physical limitations on the amount of information that can be transmitted. The use of wavelength division multiplexers (WDMS) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, such that light of two wavelengths (i.e., 1310 nm and 1550 nm) entering the inputs of the first and second fibers, respectively, are multiplexed onto a single fiber, transmitted, and then demultiplexed onto the two outputs of the first and second fibers. Light at 1550 nm is particularly desirable because minimal absorption is exhibited by optical fibers around this wavelength. Commercially available fused-fiber WDMs typically also couple and decouple light at 1550 nm and 980 nm and at 1550 nm and 1480 nm.
The principles of WDM can be extended to further increase data transmission capability by coupling additional discrete wavelengths or channels onto a single fiber using a more recent technology known as dense WDM (DWDM). Fused-fiber DWDMs may couple, for example, 8, 16, or even 32 discrete communication channels onto a single optic fiber. However, because the usable bandwidth of the light is limited, increasing the number of wavelengths necessarily results in smaller channel separation between the discrete wavelengths. In general, smaller channel spacing can be achieved by increasing the length of the fused portion of a fused-fiber DWDM. However, decreasing channel spacing presents different types of problems, such as increased sensitivity to temperature fluctuations.
As temperature increases in the fused-fiber WDM, the refractive index of the fused-fiber portion increases due to the refractive index dependence on temperature of fused silica, which is approximately 6.times.10.sup.-6 /.degree. C. This causes a longer optical path inside the coupling region of the WDM, and therefore, shifts the peak transmission wavelengths toward shorter wavelengths, thereby decreasing the channel spacing or wavelength separation capability of the WDM. These temperature-induced shifts normally do not adversely affect conventional WDMs, which typically have channel spacings of 50 nm or more. However, with DWDMs, typically having channel spacings of 1 nm or less, such wavelength shifts can pose significant problems with transmission performance.
One method to reduce temperature-induced wavelength shifts is to use a thermal-isolated device containing an electric cooler/heater with a temperature feedback element to actively maintain a constant temperature of the fused-fiber DWDM structure. However, such a device adds size, as well as cost and complexity, to the multiplexer, and thus the fiber optic communication system. Furthermore, over a wide temperature range, such as -40.degree. C. to approximately 80.degree. C., the device's ability to maintain a constant temperature decreases.
Accordingly, a structure and method are desired for controlling temperature-induced effects in fused-fiber DWDMs which overcomes the disadvantages discussed above with conventional devices.